Final answer:
A hypothesis is a tentative explanation of observations that can be tested through scientific investigation. It's the starting point for experimentation in the scientific method.
Step-by-step explanation:
The phrase that completes the sentence is hypothesis. A hypothesis is a tentative explanation of observations that acts as a guide for gathering and checking information. It's essentially a prediction that explains a phenomenon or answers a scientific question, based on prior knowledge and observation, which can be tested through further investigation and experimentation.
An effective hypothesis should be testable and falsifiable, meaning that it can be proven true or false based on evidence. It should not rely on the absence of evidence as a proof of concept. In science, forming a hypothesis is the first step in the scientific method, setting the stage for experimentation and discovery.